The present invention relates to a semiconductor device using a TFT (thin film transistor), and to a process for fabricating the same. More particularly, the present invention relates to a semiconductor device usable for a liquid crystal display device of an active matrix type.
Semiconductor devices comprising TFTs include liquid crystal display devices of active matrix type and image sensors. When exposed to air, the TFTs for use in the devices enumerated above undergo degradation due to moisture, etc., of the air. This problem is overcome by covering the TFT with a passivation film of a material containing silicon nitride as the principal component thereof. However, in case aluminum is used for the source/drain electrodes and interconnections under the passivation film, hillocks tend to generate on the surface of the aluminum film due to the heat which generates at the deposition of a passivation film. Hillocks lead to a problem of short circuit.
In the light of the aforementioned problems, an object of the present invention is to prevent hillocks from generating during formation of a passivation film by forming an anodic oxidation film on the surface of the aluminum interconnection for a thickness of from 50 to 500 xc3x85.
Contact holes are formed first on the source/drain by a known process for fabricating a TFT. Then, an aluminum film for forming an aluminum interconnection is deposited to a thickness of from 3,000 xc3x85 to 2 xcexcm, preferably, for a thickness of from 4,000 to 8,000 xc3x85. To suppress the occurrence of hillocks, the aluminum film may contain up to about 5% by weight of an impurity such as Si, Sc, and Cu.
The resulting structure is subjected to anodic oxidation in an electrolytic solution by applying a current to form a barrier type anodic oxide for a thickness of from 50 to 500 xc3x85. The barrier type anodic oxide is particularly suitable for the object of the present invention because it is hard and dense. The barrier type anodic oxide can be formed by connecting the article to the positive electrode in a proper electrolytic solution kept substantially neutral, and by applying a current while elevating the voltage.
More particularly, for instance, an L-tartaric acid diluted to a concentration of 5% by using ethylene glycol and pH-controlled to a value around 7 can be used as the electrolytic solution. The substrate is immersed into the solution, and the positive side of the constant current supply is connected to the aluminum film on the substrate while connecting the negative side to the platinum electrode. Then, oxidation is continued under voltage while maintaining the current constant, until a value in the range of from 5 to 30 V is attained. Oxidation is thereafter continued under a constant voltage until almost no current is found to flow any more. Thus is obtained an aluminum oxide film on the surface of the aluminum film. The thickness of the aluminum oxide film increases linearly with increasing voltage; thus, a thicker film is obtained for a higher voltage.
The thicker the aluminum oxide film is, the better the function as a barrier becomes. A higher voltage must be applied to obtain a thicker film. However, as the applied voltage increases, a fear of element breakdown also arises.
Thus, the voltage and the thickness for the aluminum oxide must be determined to a level as such that the element would not be destroyed.
The aluminum film and the aluminum oxide film thus obtained are etched to form an aluminum interconnection whose surface is covered by an aluminum oxide film. By forming a passivation film on the surface of the resulting structure, the generation of hillocks can be prevented by the anodic oxide film.
A passivation film can be formed without generating any hillocks on the surface of the aluminum interconnection by forming an anodic oxide film on the surface of the aluminum interconnection. Thus, failures as severe as to break the passivation film can be prevented from occurring. In this manner, a thin passivation film particularly effective in forming fine TFTs can be obtained. In case of using the TFTs in the pixel portions of a liquid crystal display device, the generation of short circuit had been feared due to the hillocks that are formed on the electrodes and interconnections, which contact with the electrodes disposed opposed thereto. However, this problem short circuit can be overcome by utilizing the present invention, and the total product yield can be improved.